The research described in this renewal application represents the continuation of our efforts on this project over the past 7 years. The central theme of our project has been the structural and functional characterization of U1-related sequences and their transcripts in the silkmoth, Bombyx mori. This scientific program has served and will continue to be a vehicle to educate and motivate promising minority students to pursue careers in the biomedical sciences. U1 RNA is one of six abundant small nuclear RNAs (snRNAs), U1-U6, which are known to associate with six to ten proteins. These RNA/protein complexes form small ribonucleoprotein particles (snRNPs) which function in splicing hnRNA into mRNA. Our laboratory has structurally characterized three categories of U1-related sequences in the silkmoth. One category is comprised of isoforms or variants capable of assembling into immunologically active U1 snRNPs. A second class is represented by pseudogenes and the third group is composed of a highly reiterated U1- related sequence family which we labelled Bm1. The U1 variants and Bm1 repetitive elements are developmentally and tissue specifically transcribed suggesting a role in control of gene expression. This proposal describes experiments designed to elucidate the role U1 and snRNP protein variants play in the control of gene expression. Their function in hnRNA splicing will be investigated using developmentally staged silk glands, follicles and several B. mori cell lines. U1 isoforms previously characterized in our laboratory will be used in reconstitution experiments in which their differential assembly into snRNP/hnRNP particles, their association with intron-containing fibroin and chorion RNAs and their splicing capacities will be ascertained. These studies should help define the suspected role of developmental and tissue-specific expression of U1 variants in differential splicing and development. Due to the universal and central nature of the questions being asked, the answers obtained during the course of these experiments may have profound biomedical implications.